Color conversion method and apparatus for chromakey processing

ABSTRACT

Upon chromakey processing, 4×4 transformation matrix is provided through which a backing color selected is transformed to the original point and monochrome color is transformed to vertically distribute at a spot away from the original point along an axis. Matrix multiplication comprising only addition and multiplication is applied to a foreground image data signal to generate a masked output image and a color-processed foreground output image without requiring complex arithmetic operations.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to color conversion method andapparatus for chromakey processing.

[0003] 2. Description of the Related Art

[0004] A chromakey apparatus or image compositing apparatus whichutilizes a color differential background (mainly in the form of blue orgreen screen) for broadcast and feature movie productions is oftendesigned to process an image to be processed in a manner describedbelow. Firstly, an object of shooting is located in front of a singlecolored screen as a background and is shot to produce an image material(hereinafter referred to as an “image to be processed”). Apart fromthis, an image material to be used as a background in composition isprepared. With these image materials being inputted to the apparatus,the following steps are carried out sequentially.

[0005] (1) One color (backing color) is selected for the background orto represent the background.

[0006] (2) Two classification boundaries are set in a color space suchas an RGB or YIQ color space to divide the color space into threedifferent regions of complete backing, complete foreground andtransition regions. The complete backing region need to contain thebacking color. The term “RGB” refers to a color system of three primarycolors which have most in common with the visible characteristics ofhuman beings. The term “YIQ” refers to a color system which is one ofso-called color difference systems adapted for TV displays andtransmittal of images; Y stands for a component representing brightnessand I and Q are orthogonal coordinates for indicating positions on acolor hue ring.

[0007] (3) Each of pixels constituting the image to be processed ischecked as to its position in the color space.

[0008] (4) When the pixel is contained in the complete backing orcomplete foreground region, its contribution is defined to be 0% or100%, respectively. If it is contained in the transition region,distances of the pixel to the two classification boundaries are checkedto determine its contribution on the basis of its proximity orsimilarity to the complete regions; the contribution is greater than 0%and smaller than 100%.

[0009] (5) From each color component of the color to be processed, eachcolor component of the backing color is subtracted and the subtractiveresult is multiplied by inverse of the contribution obtained in (4).Then, the backing color is added thereto to determine an essential colorof the object of shooting.

[0010] (6) The product of the multiplication of the essential color andthe contribution is added to the product of the multiplication of thecomposite background and the complement of the contribution, i.e.,(100%-[contribution]).

[0011] The composite result can be obtained by the above steps. In step(2) above, an RGB space with three axes of the three primary colors ofred, green and blue is popularly used in conventional apparatuses.

[0012] The conventionally used RGB space is, however, a color coordinatespace adapted to characteristics of a camera or a display and has littlecausal relation to intrinsic processing structure of chromakey. Acoordinate space should be employed which is suited for classifyingcolors and for checking distances in a color space. In other words, itis essential to use a coordinate space which contributes to easyhandling such as checking of similarity of the pixels of the image to beprocessed to the backing color or to the color of the object of shootingin the foreground.

[0013] The present invention was made in view of the above and has itsobject to provide color conversion method and apparatus for chromakeyprocessing which can produce a masked output image and a color-processedforeground output image without requiring complex arithmetic operationsin order to produce a composite image with backing color componentsbeing suppressed.

BRIEF SUMMARY OF THE INVENTION

[0014] The present invention is directed to a color conversion methodfor chromakey processing wherein a foreground image taken with an objectof shooting located in front of a single colored screen and a backgroundimage to be used in a completed composite image are composited, whichcomprises

[0015] transforming an original coordinate system into a coordinatesystem wherein a backing vector is a principal axis and a backing colorselected is at an original point, said backing vector being line drawnperpendicularly from the backing color to monochrome straight line, and

[0016] comparing a distance between pixel data to be processed and thebacking color, a distance defined by a base control variable selectedand a distance defined by a mask control variable selected, generatingtransformation coefficients on the basis of a comparison result beforepixel processing and using said transformation coefficients to performprocessing on a pixel by pixel basis, thereby generating a mask signal.

[0017] Said color conversion method for chromakey processing may furthercomprise comparing the distance between pixel data to be processed andthe backing color, the distance defined by the base control variableselected and a distance defined by a spill control variable selected,generating transformation coefficients on the basis of a comparisonresult before pixel processing and using said transformationcoefficients to perform processing on a pixel by pixel basis, therebysuppressing any spill of backing color components existing in theforeground image and replacing the same with a replacement colorselected.

[0018] The present invention is further directed to a color conversionapparatus for chromakey processing wherein a foreground image taken withan object of shooting located in front of a single colored screen and abackground image to be used in a completed composite image arecomposited, which comprises

[0019] a setup data input module for setup of a backing color, areplacement color, a base control variable, a mask control variable anda spill control variable,

[0020] a setup data translation module for transforming an originalcoordinate system into a coordinate system wherein a backing vector is aprincipal axis and a backing color selected in said setup data inputmodule is at an original point, said backing vector being line drawnperpendicularly from the backing color to monochrome straight line, forcomparing a distance between pixel data to be processed and the backingcolor, a distance defined by the base control variable selected and adistance defined by the mask control variable selected, for comparingthe distance between pixel data to be processed and the backing color,the distance defined by the base control variable selected and adistance defined the by the spill control variable selected and forgenerating transformation coefficients on the basis of comparisonresults before pixel processing and

[0021] a pixel processing module for using said transformationcoefficients generated in the setup data translation module to performprocessing on a pixel by pixel basis, thereby generating a mask signaland suppressing any spill of the backing color components existing inthe foreground image and replacing the same with a replacement colorselected.

[0022] Color conversion method and apparatus according to the inventioncan provide the following advantages.

[0023] According to the invention, two concepts are adopted:

[0024] (1) conversion to arrange a group of colors contained in theobject of shooting in the foreground to a color form close to a planetypically expressed by z=constant; and

[0025] (2) conversion to enable a distance between any selected colorand the backing color on the plane of z=constant to be dealt with anormalized numerical value. Thus, these conversions are adopted so thatthe colors of the object of shooting, which need to be treatedequivalently in the composition process, can be concentrated in a singlespot in a one-dimensional feature space along, for example, the z-axis.The z-component represents the similarity to the backing color.Moreover, the positional relationship between the color to be processedand the classification boundaries can be computed simply by comparingthem with respect to size in terms of one of the components (zcomponent) of the space coordinate. Furthermore, the proximity to theclassification boundaries can be detected simply by checking the size ofone of the components of the space coordinate.

[0026] As for internal processing, the invention provides the advantagethat the multiplication of the inverse of the contribution (or thedivision using the contribution as a divisor) in step (5) and themultiplication in step (6) can be replaced by a multiplication and anaddition by carrying out the operations in a normalized coordinatespace. This processing using the color space can be utilized as a coreto efficiently implement functional features of chromakey such ascorrection of an uneven backing, processing of a semitransparent objectof shooting and suppression of spill of the backing color to the objectof shooting in the foreground.

[0027] Now, the invention will be described in greater detail withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a schematic block diagram of an apparatus according toan embodiment of the invention;

[0029]FIG. 2 is a schematic illustration of selecting a backing color 6on an image plane;

[0030]FIG. 3 is an illustration of locations of a monochrome line 7 anda backing vector 8 in a color space;

[0031]FIG. 4 is an illustration of rotating the monochrome line 7 ontothe xy plane;

[0032]FIG. 5 is an illustration of rotating the monochrome line 7 ontothe x-axis;

[0033]FIG. 6 is an illustration of an angle by which the backing color 6is rotated onto the xz plane;

[0034]FIG. 7 is an illustration of translating the backing color 6 tothe original point;

[0035]FIG. 8 is a perspective illustration of an apparatus according tothe embodiment of the invention;

[0036]FIG. 9 is a block diagram showing a process of a pixel processingmodule 3 of the embodiment of the invention; and

[0037]FIG. 10 schematically shows a color-processed foreground outputimage and a masked output image of the embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0038] Inputted to the apparatus are signals of images comprising (a) a“foreground image” which is an image material taken with an object ofshooting being located in front of a single colored screen and (b) a“composite background image” which is an image material to be used as abackground in a completed composite image. The signals to be processedby the apparatus are RGB digital video signals and each pixel of theimages needs to hold data. As shown in FIG. 1, the chromakey apparatusaccording to the invention comprises three modules, i.e.,

[0039] (1) a setup data input module 1,

[0040] (2) a setup data translation module 2 and

[0041] (3) a pixel processing module 3.

[0042] Now, each of the modules will be described in detail.

[0043] The setup data input module 1 performs the following process.

[0044] This module deals with a user's inputting of setup data to theapparatus. The system according to the invention uses five kinds ofsetup data:

[0045] (1) A backing color 6 (see FIG. 3) which is typical.

[0046] (2) A replacement color which is selected to suppress backingcolor 6 components in the object of shooting in the foreground.

[0047] (3) A base control variable which indicates a tolerable level ofthe color feature regarded completely as backing and which representsthe location of a boundary between a complete background region and atransition region.

[0048] (4) A mask control variable which is an intensity parameter fordetermining a mask density in terms of similarity to the backing color 6and which represents the location of a boundary between the transitionregion and a complete foreground region.

[0049] (5) A spill control variable which is an intensity parameter tobe used for suppressing any spill of the backing color 6 existing in theobject of shooting in the foreground and replacing it with thereplacement color.

[0050] Of these setup data, the backing color 6 is taken into theprocessing apparatus by a user who moves an on-screen square cursor 4 onan image plane displaying a foreground image, as shown in FIG. 2, onto abacking screen 5, and depresses a decision button. The data is expressedas B=X_(b), Y_(b), Z_(b).

[0051] The replacement color is taken into the processing apparatus,using three volume control knobs for setting values for the respectivecolor components. The data is expressed as R=X_(r), Y_(r), Z_(r).

[0052] Each of the base, mask and spill control variables is taken intothe processing apparatus by the user who inputs the intensity thereof bymeans of a corresponding volume control knob. The data are expressed asI_(b), I_(m), I_(s), respectively.

[0053] The setup data translation module 2 performs the followingprocess.

[0054] In order that the data inputted to the setup data input module 1may be used for pixel processing, preparation is made for processing thedata to efficiently compute the pixel data. This preparation is referredto as “translation”.

[0055] For easy separation, upon pixel processing, of the region of thebacking color 6 from that of the object of shooting in the foreground,mapping is basically carried out to express characterizing masses of theregions by means of single variables, respectively. In this connection,a “monochrome color” is used as a typical color sample of the object ofshooting. That is , a monochrome color ranging from black to white withgradations of gray in between may possibly be the foreground against anycolored backing so that it is generally taken for a foreground color.The monochrome color is expressed in a color space by a straight lineregardless of whether the color space is that of YIQ or of RGB. Thestraight line is referred to as a “monochrome straight line” 7.Referring to FIG. 3, line drawn perpendicularly from the backing color 6inputted to the setup data input module 1 to the monochrome straightline 7 is referred to as a “backing vector” 8. The pixel data in thecolor space changes along the backing vector 8 from a color similar tothe backing color 6 to the color similar to that of the foreground.Thus, the similarity of the color data to be processed to the backingcolor 6 can be readily determined by transforming the coordinate systemto one where the backing vector 8 is a principal axis and thenperforming the process in this system.

[0056] Now, the process of the coordinate system transformation usingthe data inputted to the setup data input module 1 and specifictransformation formulae which are used for the embodiment will bedescribed in detail below.

[0057] Assume here that the original coordinate system is RGB and thecoordinate system obtained as a result of the transformation is xyz.Since the transformation is realized by synthetically combining a numberof transformations, all intermediate coordinate systems will benumerically suffixed.

[0058] First, the monochrome straight line 7 is rotated around thex-axis until it is located on the xy plane as shown in FIG. 4. Thestraight line which is a result of the monochrome straight line 7projected onto the xz plane is out of alignment from the x-axis by anangle of 45°, which is expressed by the matrix below. $\begin{matrix}{{P_{1} = {M_{1}P}}{M_{1} = \begin{pmatrix}1 & 0 & 0 & 0 \\0 & w & w & 0 \\0 & {- w} & w & 0 \\0 & 0 & 0 & 1\end{pmatrix}}{w = \frac{1}{\sqrt{2}}}} & {{formulae}\quad (1)}\end{matrix}$

[0059] where P is the original data to be processed and P_(n) is thedata to be processed as obtained by the transformation using matrixtransformation M_(n). Then, the monochrome straight line 7 on the xyplane is rotated around the z-axis until it is located on the x-axis asshown in FIG. 5. The monochrome straight line 7 rotated by M₁ is out ofalignment from the x-axis by an angle θ which is expressed by formula(2): $\begin{matrix}{\theta = {\arcsin \left( \frac{\sqrt{2}}{\sqrt{3}} \right)}} & {{formula}\quad (2)}\end{matrix}$

[0060] A rotational transformation as expressed by formulae (3) below isused for a turn of the angle θ. $\begin{matrix}{{P_{2} = {M_{2}P_{1}}}{M_{2} = \begin{pmatrix}v & v_{0} & 0 & 0 \\{- v_{0}} & v & 0 & 0 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{pmatrix}}{{v = {{\cos \quad \theta} = \frac{1}{\sqrt{3}}}},{v_{0} = {{\sin \quad \theta} = \frac{\sqrt{2}}{\sqrt{3}}}}}} & {{formulae}\quad (3)}\end{matrix}$

[0061] After the above two rotational transformations, a transformationof laying the backing vector 8 onto the z-axis is carried out while themonochrome straight line 7 is held on the x-axis. With the abovecoordinate transformations, B=X_(b), Y_(b), Z_(b) of the backing color 6in the RGB coordinate system is shifted to B₂.

B ₂ =M ₂ M ₁ B  formula (4)

[0062] Then, B₂ is rotated around the x-axis and moved to an area of z<0on the xz plane. Distance d between the projected point of B₂ on the yzplane and the original point is expressed by formula 5 below, providingthat the coordinate components of B₂ are B_(2x), B_(2y) and B_(2z), (seeFIG. 6).

d={square root}{square root over (B_(2y) ²+B_(2z) ²)}  formula (5)

[0063] The cosine and sine of an angle ψ between the projected point ofB₂ on the yz plane and the negative direction of the z-axis areexpressed by formulae (6) below. $\begin{matrix}{{{\cos \quad \psi} = \frac{B_{2y}}{d}}{{\sin \quad \psi} = \frac{- B_{2z}}{d}}} & {{formulae}\quad (6)}\end{matrix}$

[0064] Therefore, the matrix representing the rotational transformationis expressed by formulae (7) below. $\begin{matrix}{{M_{3} = \begin{pmatrix}1 & 0 & 0 & 0 \\0 & W & {- V} & 0 \\0 & V & W & 0 \\0 & 0 & 0 & 1\end{pmatrix}}{W = {\cos \quad \psi}}{V = {\sin \quad \psi}}} & {{formulae}\quad (7)}\end{matrix}$

[0065] With the above transformations, the backing color 6 is shifted toB₃ which is expressed by formula (8) below.

B ₃ =M ₃ B ₂  formula (8)

[0066] This can be translated to the original point of the xyzcoordinate system by a matrix transformation using formula (9) below(see FIG. 7). $\begin{matrix}{M_{4} = \begin{pmatrix}1 & 0 & 0 & {- B_{2x}} \\0 & 1 & 0 & 0 \\0 & 0 & 1 & 1 \\0 & 0 & 0 & 1\end{pmatrix}} & {{formula}\quad (9)}\end{matrix}$

[0067] Now, P₂ can be transformed as formula (10) below, using M₃ andM₄.

P ₄ =M ₄ M ₃ P ₂  formula (10)

[0068] With the above transformations, the monochrome straight line 7 isshifted to be expressed by formulae (11) below. $\begin{matrix}\left\{ \begin{matrix}{\quad {x = t}} \\{\quad {y = 0}} \\{\quad {z = {- B_{3z}}}}\end{matrix} \right. & {{formulae}\quad (11)}\end{matrix}$

[0069] The coordinate system obtained as a result of the abovetransformations shows the following two characteristics: when the pixeldata to be processed is converted into this coordinate system,

[0070] (1) the pixel is close to the backing color 6 as the z componentis close to 0 and

[0071] (2) the pixel is close to the monochrome color as the z componentis close to −B_(3z).

[0072] Thus, the similarity of the pixel to be processed to the backingcolor 6 can be determined simply by checking the z component.

[0073] Finally, the coordinate system is transformed into a normalizedcoordinate system, using the base, mask and spill control variables.

[0074] The base control variable is used to give a tolerable level tothe color selected as the backing color 6. The mask control variabledefines the distance between the backing color 6 and the color of thecomplete foreground region. The spill control variable defines thedistance between the backing color 6 and the color of the spill regions(i.e., the region in the foreground which is affected by the backingcolor 6).

[0075] Assume that the distance between the pixel data being processedand the backing color 6 data is l. Then, the distance defined by thebase control variable is l_(b), the distance defined by the mask controlvariable is l_(m) and the distance defined by the spill control variableis l.

[0076] If l is smaller than l_(b), the data is regarded as that of thecomplete background region. If l is greater than l_(m), the data isregarded as that of the complete foreground. If l is greater than l_(b)and smaller than l_(m), outputted is a mask signal showing anintermediary value between 0% and 100% and depending upon whether andhow l is close to l_(b) or l_(m). Formula (12) used to compute a masksignal a in the transition region is: $\begin{matrix}{a = \frac{l - l_{b}}{l_{m} - l_{b}}} & {{formula}\quad (12)}\end{matrix}$

[0077] As to spill control, the following process is used. If l issmaller than l_(b), the data is not processed at all since it is of thecomplete background. If l is greater than l_(s), it is assumed to haveno influence by the backing color 6; then, the data of the originalforeground image is outputted. If l is greater than l_(b) and smallerthan l_(s), the degree of influence of the backing color 6 variesdepending on whether and how l is close to l_(b) or l_(s); in thisregion, the spill intensity parameter s determined by formula 13 belowis used to process the foreground data on a pixel by pixel basis.$\begin{matrix}{s = \frac{l_{s} - l}{l_{s} - l_{b}}} & {{formula}\quad 13}\end{matrix}$

[0078] The coordinate system transformation is made to incorporatenormalization of the coordinate system including this process so as toprevent the above two divisions from being performed on a pixel by pixelbasis. First, a point separated by l_(b) in the direction of the z-axisfrom the backing color 6 (that has been moved to the original point bythe transformations conducted so far) is moved to the original point.$\begin{matrix}{{P_{5} = {M_{5}P_{4}}}{M_{5} = \begin{pmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & 1 & {- l_{b}} \\0 & 0 & 0 & 1\end{pmatrix}}} & {{formulae}\quad (14)}\end{matrix}$

[0079] Then, a scale transformation is performed to make l_(m) equal to1.0, while maintaining the original point in the current position.$\begin{matrix}{{P_{6} = {M_{6}P_{5}}}{M_{6} = \begin{pmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & \frac{1}{l_{m} - l_{b}} & 0 \\0 & 0 & 0 & 1\end{pmatrix}}} & {{formulae}\quad (15)}\end{matrix}$

[0080] In the coordinate system P₆ obtained by the transformations M₅and M₆ above, the value of a agrees with z within the range of 0<z<1.Additionally, a=0 when z≦0; and a=1 when z≧1. When the monochrome colorand the data which is far from the backing color 6 are used for theforeground, l_(m) agrees with −B_(3z).

[0081] Meanwhile, a scale transformation for making l_(s) equal to 1.0is independently performed after the transformation using M₅, whilemaintaining the original point in the current position: $\begin{matrix}{{P_{7} = {M_{7}P_{5}}}{M_{7} = \begin{pmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & \frac{1}{l_{b} - l_{s}} & 0 \\0 & 0 & 0 & 1\end{pmatrix}}} & {{formulae}\quad (16)}\end{matrix}$

[0082] In the coordinate system P₇ obtained by the transformations M₅and M₇ above, the value of s is expressed by s=1−z where z is in therange of 0<z<1. Additionally, s=1 when z≦0; and s=0 when z≧1.

[0083] It is the role of the setup data translation module 2 to computethe transformation M which is a composite transformation using M₁through M₆ and the transformation N which is a composite transformationusing M₁ through M₅ and M₇:

M=M ₆ M ₅ M ₄ M ₃ M ₂ M ₁

N=M ₇ M ₅ M ₄ M ₃ M ₂ M ₁  formulae (17)

[0084] Of each of the composite matrixes, only the part corresponding tothe z component of the third row is used for the processing. If theinput data is r, g and b, required computations are expressed byformulae 18 below. $\begin{matrix}{\begin{matrix}{a = \quad \left( {{{- 2}v\quad w\quad {V \cdot r}} + {\left( {{v\quad w\quad V} - {w\quad W}} \right) \cdot g} + {\left( {{v\quad w\quad V} + {w\quad W}} \right) \cdot}} \right.} \\{{\left. \quad {b + d} \right) \cdot \frac{1}{l_{m} - l_{b}}} + \frac{- l_{b}}{l_{m} - l_{b}}}\end{matrix}\begin{matrix}{s = \quad \left( {{{- 2}v\quad w\quad {V \cdot r}} + {\left( {{v\quad w\quad V} - {w\quad W}} \right) \cdot g} + {\left( {{v\quad w\quad V} + {w\quad W}} \right) \cdot}} \right.} \\{{\left. \quad {b + d} \right) \cdot \frac{1}{l_{b} - l_{s}}} + \frac{- l_{b}}{l_{b} - l_{s}}}\end{matrix}} & {{formulae}\quad (18)}\end{matrix}$

[0085] The pixel processing module 3 is adapted to perform the followingprocess.

[0086] The input data is processed on a pixel by pixel basis by usingthe transformation coefficients generated by the setup datatransformation module 2. The input color data of the foreground image istransformed by means of the transformation coefficients and the zcomponent of the data coordinate obtained is checked. Then, the originalcolor data is processed for color by using the value obtained. If theresult obtained from the input data P by the transformation M is Q andthe result obtained from the input data P by the transformation N is K,their z coordinates a and s are required.

Q=MP

a=Q _(z)

K=NP

s=K _(z)  formulae (19)

[0087] First, the value A of the mask signal to be outputted is obtainedby clamping a within the range between 0 and 1. $\begin{matrix}{A = \left\{ \begin{matrix}0 & \left( {a < 0} \right) \\a & \left( {0 \leqq a \leqq 1} \right) \\1 & \left( {a > 1} \right)\end{matrix} \right.} & {{formula}\quad (20)}\end{matrix}$

[0088] Then, the following computations are used for the foregroundwhich has been processed for color. Assume that the result obtained byclamping s within the range between 0 and 1 is S. S shows the extent ofinfluence of the backing color 6 on the color of the object. The pixelcolor is changed as a function of the extent. If the influence of thebacking color 6 is 100%, the color of the object is replaced by thereplacement color. If the influence of the backing color is 0%, thecolor of the object is outputted without modification. If the influenceis somewhere in between, the color of the object is brought closer tothe replacement color as a function of the percentage. The resultobtained as a result of the above processing is referred to as“processed foreground” U, which is derived from formulae (21) below.

U=(R−B)×S+P

X _(u)=(X _(r) −X _(b))·S+X _(p)

Y _(u)=(Y _(r) −Y _(b))·S +Y_(p)

Z _(u)=(Z _(r) −Z _(b))·S +Z _(p)  formulae (21)

[0089] With A and U being obtained as a result of the above process, theforeground image is combined with the background image for composition,using a general linear key composite means. If the background image forcomposition is G(X_(g), Y_(g), Z_(g)) and the end result is V(X_(v),Y_(v), Z_(v)), the end result is obtained by formulae 22 below.

X _(v) =X _(u) ·A+X _(g)·(1−A)

Y _(v) =Y _(u) ·A+Y _(g)·(1−A)

Z _(v) =Z _(u) ·A+Z _(g)·(1−A)  formulae (22)

[0090] There may be provided a wide variety of apparatuses which can beused for carrying out the method of the invention; a typical one will bedescribed below.

[0091] Referring to FIG. 8, the apparatus comprises three modules, i.e.,a setup data input module 1, a setup data translation module 2 and apixel processing module 3. The translation module 2 is in the form of apersonal computer that performs computations for the purpose of setupdata translation and, at the same time, carries out other operationssuch as taking in data from the setup data input module 1 andtransferring the outcome of translation to the registers of the pixelprocessing module 3. The input module 1 is connected with thetranslation module 2 through an RS232C serial cable 9 whereas theprocessing module 3 is connected with the translation module 2 through aPCI interface.

[0092] The setup data input module 1 operates in a manner as describedbelow. The user inputs the parameters of

[0093] (1) backing color 6,

[0094] (2) replacement color,

[0095] (3) base control variable,

[0096] (4) mask control variable and

[0097] (5) spill control variable.

[0098] As shown in FIG. 8, the input module 1 has eight volume controlknobs and a push button.

[0099] The volume control knobs #1 and #2 are used to control thelocation of the cursor 4 shown on the picture plane (see FIG. 2). As thebutton is depressed, the pixel data (RGB) of the spot where the cursor 4is located is taken in from the frame buffer arranged in the pixelprocessing module 3 and transferred to the personal computer of thesetup data translation module 2 so that it is used as a backing color 6parameter.

[0100] The volume control knobs #3, #4 and #5 are used for inputting thereplacement color. The knobs #3, #4 and #5 correspond to the R, G and Bcomponents, respectively. The inputted data is transferred to thepersonal computer of the setup data translation module 2 through theRS232C serial cable 9.

[0101] The volume control knob #6 is used to input the base controlvariable which is transferred to the personal computer of the setup datatranslation module 2 through the RS232C serial cable 9.

[0102] The volume control knob #7 is used to input the mask controlvariable which is transferred to the personal computer of the setup datatranslation module 2 through the RS232C serial cable 9.

[0103] The volume control knob #8 is used to input the spill controlvariable which is transferred to the personal computer of the setup datatranslation module 2 through the RS232C serial cable 9.

[0104] The setup data translation module 2 operates in a manner asdescribed below.

[0105] This module 2 translates the parameters inputted by means of thesetup data input module 1 and transfers the obtained results to theregisters of the pixel processing module 3. Since composite elements areutilized in the pixel processing module 3, coefficients listed informulae (23) below, which have been already shown in formula (18), arecomputed and stored, separately. $\begin{matrix}{{K_{0} = {{- 2}v\quad w\quad {V \cdot r}}}{K_{1} = {{v\quad w\quad V} - {w\quad W}}}{K_{2} = {{v\quad w\quad V} + {w\quad W}}}{K_{3} = d}{K_{4} = \frac{1}{l_{m} - l_{b}}}{K_{5} = \frac{- l_{b}}{l_{m} - l_{b}}}{K_{6} = \frac{1}{l_{b} - l_{s}}}{K_{7} = \frac{- l_{b}}{l_{b} - l_{s}}}{K_{8} = X_{r}}{K_{9} = Y_{r}}{K_{10} = Z_{r}}} & {{formulae}\quad (23)}\end{matrix}$

[0106] The total of eleven computed outcomes are expressed in a fixedpoint format and transferred to the respective coefficient registers inthe pixel processing module 3.

[0107] The pixel processing module 3 operates in a manner as describedbelow.

[0108] Referring to FIG. 9, the pixel processing module 3 comprises anadder 10, a multiplier 11, a clamping circuit 12 and an inverter 13.Each of the components of the inputted RGB digital signal is multipliedby coefficients K₀, K₁ and K₂ and added to each other along with K₃.Then, the outcome is multiplied by a coefficient K₆, and K₇ is addedthereto. Thereafter, the outcome is clamped within the range between 0and 1 to output the mask signal. Similarly, each of the components ofthe input RGB digital signal is multiplied by coefficients K₀, K₁ and K₂and added to each other along with K₃. Then, the outcome is multipliedby a coefficient K₄, and K₅ is added thereto. Thereafter, the outcome isinverted and a constant 0×400 is added thereto before it is made to passthrough the clamping circuit 12 to obtain a result A. Then, A ismultiplied by a coefficient K₈, and R of the input signal is addedthereto. The sum is clamped to produce an R output. Similarly, A ismultiplied by a coefficient K₉, and G of the input signal is addedthereto. The sum is clamped to produce a G output. Finally, A ismultiplied by a coefficient K₁₀, and B of the input signal is addedthereto. The sum is clamped to produce a B output. The constant 0×400 isa hexadecimal representation of the internal representation 1.0.

[0109]FIG. 10 shows a masked output image and a color-processedforeground output image which may be outputted from the pixel processingmodule 3.

[0110] Thus, according to the invention, for chromakey processing, themasked output image and the color-processed foreground output image canbe generated without complex arithmetic operations when preparing thecomposite image with the backing color components being suppressed.

[0111] It is to be understood that the color conversion method andapparatus for chromakey processing according to the invention are notlimited to the illustrated embodiment and that various changes andmodifications may be made without departing from the scope and spirit ofthe invention.

[0112] Thus, the color conversion method and apparatus for chromakeyprocessing according to the invention provide a remarkable advantagethat, for chromakey processing, the masked output image and thecolor-processed foreground output image can be generated without complexarithmetic operations when preparing the composite image with thebacking color components being suppressed.

What is claimed is:
 1. A color conversion method for chromakeyprocessing wherein a foreground image taken with an object of shootinglocated in front of a single colored screen and a background image to beused in a completed composite image are composited, which comprisestransforming an original coordinate system into a coordinate systemwherein a backing vector is a principal axis and a backing colorselected is at an original point, said backing vector being line drawnperpendicularly from the backing color to monochrome straight line, andcomparing a distance between pixel data to be processed and the backingcolor, a distance defined by a base control variable selected and adistance defined by a mask control variable selected, generatingtransformation coefficients on the basis of a comparison result beforepixel processing and using said transformation coefficients to performprocessing on a pixel by pixel basis, thereby generating a mask signal.2. A color conversion method according to claim 1 further comprisingcomparing the distance between pixel data to be processed and thebacking color, the distance defined by the base control variableselected and a distance defined by a spill control variable selected,generating transformation coefficients on the basis of a comparisonresult before pixel processing and using said transformationcoefficients to perform processing on a pixel by pixel basis, therebysuppressing any spill of backing color components existing in theforeground image and replacing the same with a replacement colorselected.
 3. A color conversion apparatus for chromakey processingwherein a foreground image taken with an object of shooting located infront of a single colored screen and a background image to be used in acompleted composite image are composited, which comprises a setup datainput module for setup of a backing color, a replacement color, a basecontrol variable, a mask control variable and a spill control variable,a setup data translation module for transforming an original coordinatesystem into a coordinate system wherein a backing vector is a principalaxis and a backing color selected in said setup data input module is atan original point, said backing vector being line drawn perpendicularlyfrom the backing color to monochrome straight line, for comparing adistance between pixel data to be processed and the backing color, adistance defined by the base control variable selected and a distancedefined by the mask control variable selected, for comparing thedistance between pixel data to be processed and the backing color, thedistance defined by the base control variable selected and a distancedefined the by the spill control variable selected and for generatingtransformation coefficients on the basis of comparison results beforepixel processing and a pixel processing module for using saidtransformation coefficients generated in the setup data translationmodule to perform processing on a pixel by pixel basis, therebygenerating a mask signal and suppressing any spill of the backing colorcomponents existing in the foreground image and replacing the same witha replacement color selected.